The reaction between a metal and an organic halogen compound

1 The reaction between a metal and an organic halogen compound 

The reaction between a metal and an organic halogen compound provides a fairly general route to organometallic compounds. One of the earliest organometallic compounds ever to be prepared, ethylzinc iodide, was made in this way from zinc/copper couple and ethyl iodide. 

These data show that, apart from the elements C, N and 0 and the B-elements of the last row of the Periodic Table (Hg, Tl, Pb and Bi), the reaction between an element and methyl chloride to yield the chloride and methyl of the element is exothermic, and therefore might be expected to provide a method of synthesizing the methyls. In nearly all the examples listed where a negative value of AH is found, such reactions have been observed and many are important synthetic routes. The mechanisms of the reactions and the conditions under which they occur, however, vary greatly with different elements. Kinetic as well as thermodynamic factors are thus significant in controlling the reactions. 

In the B-elements the strength of the M—C bonds falls sharply on going from the elements of the second long Period (e.g. Sn, Sb) to those of the third long Period (e.g. Pb, Bi). The alkyls of Hg, T1 and Bi are all strongly endothermic compounds and their unfavourable heats of formation are not outweighed by the formation of the exothermic halides when the elements are treated with alkyl halides. 

The reaction between metals and organic halides is often suitable for the synthesis of organometallic compounds of the most electropositive elements. On a laboratory scale, the derivatives of Li. Mg and Al, which are very important in synthetic work, are usually prepared in this way. Some illustrations of the method are given briefly here more detailed discussion of the preparation and properties of the compounds involved appears in Chapter 3. 

---